The present invention relates to a rotary actuator including a combination of a motor and a reduction gear unit, and more particularly to a rotary actuator including a motor and a reduction gear unit disposed adjacently in the axial direction to realize a compact and slim structure. The Japanese patent application Laid-open No. 2004-69021 discloses a conventional rotary actuator including a motor and a reduction gear unit disposed adjacently in the axial direction.
In general, improvement of output torque in the rotary actuator will be realized:                (1) when the motor size is enlarged;        (2) when the motor winding is enhanced (i.e. the ampere-turn is increased);        (3) when the permanent magnet, if required for the motor, is made of a material of a higher grade;        (4) when the motor core (stator core, and rotor core) is made of a material of a higher grade; and        (5) when a reduction gear ratio of the reduction gear unit is increased.        
However, the above-described (1) to (5) cases will cause the following problems:                (1′) the overall size of the rotary actuator will increase in accordance with increasing size of the motor;        (2′) enhancing the motor wiring will increase electric power consumption and expand the motor size and the rotary actuator size;        (3′), (4′) the motor costs will increase when the used materials are higher grades, and accordingly the cost of the rotary actuator will increase correspondingly; and        (5′) increasing the reduction gear ratio will deteriorate the response of the rotary actuator and also enlarge the sizes of the reduction gear unit and the rotary actuator.        
As described above, the conventional techniques for improving the output torque of the rotary actuator tend to cause various demerits.
Furthermore, if the output torque is increased, bearings in the rotary actuator loads will be subjected to larger loads. In other words, improvement of the bearing load capacity is required. This will result in expansion in the overall size of the rotary actuator. Especially, in the case that the rotary actuator is used for the shift-range switching apparatus of an automotive transmission, there is no sufficient space available for installing a large rotary actuator in an automotive vehicle. Thus, the rotary actuator must be compact or slim. Meanwhile, the rotary actuator is required to generate a large output torque. However, simultaneously satisfying a requirement of generating a higher output and a requirement of downsizing the rotary actuator was conventionally difficult.